A well has one or more casings installed in a borehole to reinforce and seal it, and wellhead components install at the surface above the exposed end of the casings. For example, FIG. 1 shows a wellhead arrangement according to the prior art. Typically, an outermost conductor pipe is driven into place in the borehole, and a base assembly 11 is on the exposed end of conductor pipe 10. A casing head 30 lands on the base assembly 11 and connects to a surface casing 14 supported inside the conductor pipe 10. Typically, the space between the casing 14 and pipe 10 is filled with cement. One or more adapters 40 and components 50 of a blow-out preventer can connect above the casing head 30. As shown in FIG. 1, flanged connections are typically used to connect the various components together.
Because various operations may be performed at the wellhead, the arrangement of components may be modified to accommodate different operations, pressures, and implementations. One typical wellhead operation involves fracing. According to conventional practices, an isolation tool, such as a stinger, installs in the wellhead to isolate bores and outlets from pressures that may be higher than pressure-ratings for the wellhead's flange connections.
In one typical wellhead arrangement, a casing hanger supports casing in the casing head, and a tubing spool nipples to the casing head so that another hanger can be used in the tubing spool to support tubing in the casing. Typically, an adapter must be installed on the casing head prior to nippling up the tubing spool so that the adapter can pack-off or seal around the casing hanger. In another typical wellhead arrangement, a fluted casing hanger support casing in the casing head so that drilling fluids during cementing operations are allowed to return through the hanger's flutes. Later, a pack-off bushing installs above the fluted casing hanger to seal off the flutes. Typically, there is only a limited ability to test the seal created by such a pack-off.